Electronic light amplifier



April 29, 1952 DE FOREST ETAL 2,594,740

ELECTRONIC LIGHT AMPLIFIER Filed Feb. 17, 1950 70 7a 7a 74 60 f INVENTORS. L55 4): FOREST WILLIAM A. RHODES BY THE/l? HTTO RNEKS. H19 RIP/S, Mac/4, Fos r52 & HARE/5 Patented Apr. 29, 1952 ELECTRONIC LIGHT AMPLIFIER Lee de Forest, Los Angeles, Qalif., and William A. Rhodes, Phoemx, Ariz.

Application February 17, 1950, Serial No. 144,614

Claims.

1 Our invention relates to a device for increasing or amplifying the intensity of radiation and a primary object of the invention is the provision of such an amplifier. While not limited to radiation within the visible range, the invention finds particular utility in an amplifier for visible radiation and will be considered primarily in connection with visible radiation hereinafter as a matter of convenience.

An important object of the invention is to provide a light amplifier which receives a beam of light forming an image, transforms the beam of light into a beam of electrons duplicating the original image, increases the intensity of the electron beam to obtain an amplified electron beam still duplicating the original image, and which transforms the amplified electron beam into an amplified light beam duplicating the original image with increased intensity or brightness.

Expressed somewhat differently, an important object of the invention isto provide a light amplifier which receives a ray of light from an element of an image, transforms the ray of light into a ray of electrons, increases the intensity of the original ray of electrons to obtain an amplified ray of electrons, and which transforms the amplified ray of electrons into an amplified ray of light corresponding to the original ray. A related object is to provide a light amplifier of this character having means for preventing lateral spreading of the original and amplified electron rays from each element of the original image to prevent distortion in the amplified image.

More particularly, an object of the invention is to provide an amplifier for light, or other radiation, which includes a radiation-sensitive and electron-emissive element for transforming a beam or ray of radiation into a beam or ray of electrons, one or more electron-sensitive and electron-emissive elements having the characteristic of emitting more than one electron for each electron impinging thereon,for amplifying the electron beam or ray, and an electron-sensitive' and radiation-emissive element for trans forming the amplified electron beam or ray into an amplified radiation beam or ray.

Radiation-sensitive and electron-emissive materials which are sensitive to visible radiation in particular are customarily referred to as photoemissive, which term will be applied to such materials hereinafter for convenience in disclos-- ing the invention. Electron multiplying materials, i. e., electron-sensitive and electronemissive materials having the characteristic of emitting more than one electron for each electron impinging thereon, are customarily referred to as fsecondary emissive and will be referred to as such hereinafter. Materials which are electronsensitive and radiation-emissive, particularly with respect to visible radiation, are customarily rather loosely referred to as fluorescentfi More 2 accurately, such electron-sensitive and radiation emissive materials are referred to as cathodoluminescent, denoting luminescence resulting from electron bombardment, and this term will be used hereinafter to designate such materials. Cathodoluminescent materials may be divided into two groups, viz., those which are cathodofluorescent, and those which are cathodophosphorescent. In the former, luminescence ceases almost immediately upon removal of the excitation, whereas in the latter, luminescence persists after removal of the excitation. As will become apparent, the present invention finds particular utility with respect to cathodofluorescence, but since it may in some instances have utility with respect to cathodophosphorescence as well, the generic term cathodoluminescence will be employed hereinafter. Authority for the foregoing terminology is Television by V. K. Zworykin and G. A. Morton, published in 1940 by John Wiley & Sons, Inc.

The authors of Television, supra, discuss in detail numerous photoemissive, secondary emissive and cathodoluminescent materials, which materials are suitable for the hereinafter-discussed photoemissive, secondary ,emissive and cathodoluminescent elements of the light amplifier of the present invention. Consequently, it is thought unnecessary to discuss such materials herein and reference is hereby made to Television, supra, in connection with such materials.

Reference will now be had to the accompanying drawing, which discloses various embodiments of the light amplifier of the present invention and various possible applications thereof. In the drawing:

Fig. 1 is a semidiagrammatic, elevational view of a light amplifier which embodies the invention;

Fig. 2 is a semidiagrammatic, sectional view taken along the brokenline 2-2 of Fig. 1; V

Fig. 3 is a fragmentary sectional view similar to Fig. 2 but illustrating another embodiment of the invention; 7

Fig. 4 is an enlarged, fragmentary sectional view duplicating a portion of Fig. 3;

Fig. 5 is an enlarged, fragmentary sectional view taken along the broken line 5-5"of Fig. 4;

Fig. 6 is a diagrammatic view illustrating an application of the invention to a television projection system; and,

Fig. 7 is a diagrammatic view illustrating an-I other application of the invention to a television projection system. g

In its preferred embodiments, the present in vention provides an amplifier cell of laminated construction which, as will be discussed in detail hereinafter, is composed of extremely thin, superimposed laminae, one advantage of this pre-. ferred construction being that it provides a direct and extremely short light-electron-lightjpath, thereby minimizing dispersion losses ,and the like. Referringparticularly to Figs. 1 and 2, the numeral l designates a light amplifier. of the invention which includes such a laminated amplifier cell H, the latter being of circular outline in the particular construction illustrated. The amplifier cell includes a transparent lamina l2 which may be formed of glass, transparent plastic, or other suitable transparent material, the principal function of the transparent lamina |2 being to protect and support the remaining laminae of the amplifier cell and to provide a more rigid cell. Superimposed on the transparent lamina I2 is a lamina l3 of photoemissive material, the photoemissive lamina I3 preferably being, carried by the transparent lamina, as by coating the transparent lamina with photoemissive material in any suitable manner. With this construction, a. ray of light, indicated by the arrow M, which impinges on the transparent lamina i2 is transmitted thereby and, in turn, impinges on. the. photoemissive lamina l3. Consequently,.the light ray l4 excites the photoemissive material'forming the lamina l3 to an extent proportional to the intensity of the light ray, whereupon the photoemissive lamina emits electrons in proportion to the intensity of the light ray. Such emitted electrons tend to follow a path which is an extension of the path of the light ray M and are further confined to such a path, as'willbe discussed in more detail hereinafter. Thus, it willbe seen that the light ray H is transformed into an electron ray which is, in effect, an extension of the light ray.

In the preferred construction of the amplifier cell I |,'a"n extremely thin, electrically conductive lamina I5 is superimposed on the photoemissive lamina I3 and is subjected to a small positive potential, as by connecting it through a conductor Hi to a point I! on a potential divider |8 which is connected across 'a battery IE1, or other source of potential, the negative terminal of the battery being grounded. We have discovered that, with this construction, the intensity of the electron ray emitted by the photoemissive lamina I3 is materially increasedand substantially all of the emitted electrons are propelled through the conductive lamina l5 by the positive charge applied thereto, the positively-charged conductive lamina comprising an anode. As will be discussed in more detail hereinafter, various materials may be employed for the conductive lamina l5, this lamina being exemplified in Fig. 2 as comprising a thin sheet of metal, such as metallic foil. Aluminumfoil may be employed, for example.

Superimposed on the conductive lamina I5 is a laminated element which includes a secondary emissive lamina 2| and an electrically conductive lamina 22, the former being disposed adjacent and in contact with the first conductive lamina H5 in the'series. With this construction, the electron ray emitted by the photoemissive lamina l3 and accelerated and intensified by the positively charged conductive lamina l5 excites the secondary emissive lamina 2| to produce secondary emission from the lamina 2| proportional to the emission from the lamina l3, but multiplied. The secondary emission from the lamina 2| is accelerated and propelled through the conductive lamina 22 by positively charging the latter to a higher degree than the conductive lamina |5, as by connecting the conductive lamina 22 through a'conductor 23 to a point 24 on the potential divider 3 which is at a higher positive potential than the point H to which the conductive lamina I5 is connected. The accelerated secondary emission from the secondary emissive lamina 2| also tends to follow a path which is an extension of the path of the light ray I4 and, as will be discussed in more detail hereinafter, is further confined to such a path. Thus, the laminated element comprising the secondary emissive lamina 2| and the positively-charged conductive lamina 22 produces an amplified electron ray which is, in eifect, an extension of the light ray I4, the degree of amplification of the original electron ray emitted by th photo-emissive lamina |3 depending upon the secondary emission ratio of the secondary emissive material forming thelamina 2|. As discussed in "Television supra, the secondary emission ratio depends ona number of factors, such as the secondary emissive material employed, the positive potential to which the secondary emissive lamina 2| is subjected by the positively-charged conductive lamina 22, .etc. As discussed in Television, supra, the secondary emission ratio for a particular secondary emissive material is a maximum for a particular positive potential and the positive potential applied to the secondary emissive lamina 2| is preferably in the vicinity of that producing themaximum secondary emission ratio.

Preferably, the secondary emissive lamina 2| is formed by coating the conductive lamina 22 with a secondary emissive material in any suitable manner, as by spraying, distillation in a vacuum, or otherwise. However, the secondary emissive lamina may be formed in other ways, at least one of which will be discussed hereinafter. If further amplification of the once-amplified electron ray emitted by the secondary emissive lamina 2| and accelerated by the conductive lamina 22 is desired, it may be amplified any desired number of tiniesthrough the employment of additional laminated elements each similar to the laminated element comprising the laminae 2| and 22. In the particular construction illustrated, two such laminated elements are superimposed on th laminated element comprising the laminae 2| and 22, although any desired number of such laminated elements may beemployed. The laminated element which is superimposed on the laminated element comprising the laminae 2| and 22 includes a secondary emissive lamina 25 carried by an electrically conductive lamina 26,. the lamina.25 being disposed adjacent the lamina 22. The last laminated element in the series comprises a secondary emissive lamina 21 carried by an electrically conductive lamina 28,-the lamina 21 being disposed adjacent and in contact with the lamina. 25'. The conductive laminae 26 and 28 are connected to points 29 and 30, respectively, on the potential divider H! by conductors 3| and 32, respectively, the point 29-being more positive than the point .24 and the point 30 being more positive than. the point 29 so that the positivelycharged' conductive laminae, or anodes, 26 and 28 serve to propel the increasingly amplified electron ray corresponding to the light ray i4 through the amplifier cell Ordinarily, the potential diiferences between the laminae I5, 22, 26 and 28 may be of the order of magnitude of from ten to two hundred volts. The increasingly amplified electron ray. tends to follow a path which is an original electron ray corresponding to the light ray 14 many times over, the degree of amplification depending upon the secondary emission ratios of the secondary emissive laminae at the potentials applied thereto, and depending upon the number of stages of amplification, 1. e., depending upon the number of secondary emissive laminae and associated conductive laminae employed. If we assume a secondary emission ratio of five, for example, for each of the secondary emissive laminae 2|, 25 and 21 at the particular potentials applied thereto, it will be apparent that, with the three stages of amplification shown, the intensity of the electron ray emitted by the last secondary emissive lamina 21 in the series approaches one hundred and twenty-five times that ofxthe original electron ray emitted by the photoemissive lamina it. As indicated, this amplification factor of one hundred and twenty-five 'is merely illustrative since secondary emission. ratios as high as tenor more are possible.

In order to transform the amplified electron ray emitted by the last secondary emissive lamina 21in the series into a light ray, we superimpose on the last conductive lamina in the series, as by coating it with a cathodoluminescent material, for example, a cathodoluminescent lamina 40 which transforms the amplified electron ray into a light ray 4| aligned with the original light ray 14, but of greatly increased intensity, the ratio of the-intensities of the light rays 4| and I4 depending upon the over-all amplification to which the original electron ray emitted by the photoemissive lamina l3 has been subjected. If, by way of an example, we assume an over-all amplification factor of one hundred twenty-five as in the preceding paragraph, then the brightness of the amplified light ray 4| approaches one hundred twenty-five times that of the original light ray [4. Thus, it will be seen that the amplifier cell ll transforms the ray of light M, or other radiation, into a ray of electrons, amplifies the ray of electrons, and transforms the amplified ray of electrons, into an amplified ray of light. If, instead of considering only rays of light and electrons, we consider beams composed of an infinite number of rays, then an imageforming beam impinging upon the amplifier cell II is transformed into an image-duplicating beam of electrons, which beam of electrons is amplified and then transformed into an amplifled, image-duplicating beam of light of an intensity many times that of the original beam of light. Possible applications of this concept will be considered in more detail hereinafter.

In order to prevent lateral spreading of the' individual electron rays emitted by the various laminae l3, 2|, 25 and 21, we may encirclethe amplifier cell II with a helical coil 45, the axis of which is parallel to the desired paths for the light-electron-light rays. The coil 45 is connected in series with a battery 46 to produce an electromagnetic field having lines of force which extend in the directions of the desired lightelectron-light paths, 1. e., which extend in diinated, with the result that the image defined by" the amplified light beam is an excellent reproduction of the original image, except that it is many times brighter.

Although the final lamina of the amplifier cell II is illustrated as being the cathodoluminescent lamina 40, it will be understood that a final transparent lamina, not shown, similar. to thetransparent lamina l2, may be superimposed on' the cathodoluminescent lamina 40 so that the intervening laminae between the initial trans parent lamina l2 and the final transparent lamina are protected thereby. If desired,rthecathodoluminescent lamina 40 may be formed bycoating sucha final transparent lamina with "acathodoluminescent material, or by coating'th'e final conductive lamina 28 with such m'aterial. 1- For convenience in illustrating the amplifier; cell l I in the drawing, the thicknesses of the various laminae thereof have been exaggerated tremendously. Actually, each of the laminae 13.. l5,'2l, 22, 25, 26, 21 and 28 is of the order of magnitude of from 0.0001 to 0.001 inch in-thickness. The over-all thickness of the particular] amplifier cell ll illustrated is of the orderof' magnitude of 0.01 inch to 0.1 inch. The thinner-"- the amplifier cell II, the smaller is the tendency of the electron rays to spread laterally and the previously discussed means for confining the electron rays to pre-determined paths may be eliminated by making the amplifier cell suflicient- 1 1y thin. It will be understood that, in order for the photoemissive lamina I3 and the secondary emissive laminae'2l, 25 and 21 to be stable electronsources, some means must be provided forreplacing the electrons emitted thereby. In the: particular construction illustrated, such a means is provided by the electric circuit interconnect-' ing these laminae. It should be pointed out that instead'of charging the conductive" laminae I5, 22, 26 and-28 to" progressively higher positive potentials from a direct current source, we may employ a veryhigh frequency're'ctified current so that a series I'- of rectified positive pulses is applied to the con ductive laminae. In television applications of the amplifier cell II, which will be discussed iii more detail hereinafter, the frequency of such rectified positive pulses is preferably sufficiently higher than the maximum video frequencies employed in television receivers so as to avoidp'ulsa tion eifects in the amplified image.

It will be noted that no envelope is shown enclosing theamplifier cell i I, it being unnecessary to use such an envelope. .1 Referring now to Figs. 3 to 5 of the drawing.- we show a laminated amplifier cell 50 which in--' cludes a transparent lamina 5| corresponding to the transparent lamina l2 of the amplifier cell II, and which includes a transparent lamina 52. Superimposed on the transparent lamina 5! is a photoemissive lamina 53, and superimposed upon the photoemissive lamina are superimposed,- laminated elements 54,255 and 5B which will be discussed in more detail in the following para-': graph. Superimposed on the laminated element 56 is a cathodolminescent lamina 5'! correspond-;;- ingto the cathodoluminescent lamina 40 of-the amplifier cell II, the transparent lamina 52 be-,, ing superimposed upon the cathodoluminescent lamina 51. Referring particularly to Figs. 4gand 5, the laminated element 54, which is typical of the elements 54, 55 and 56,,comp'rises an electrically.

area-rat:

onductiyelaminafifl and .asecondary emissive. lamina. 65!, thelaminae. 60-. and BI being; substantiallyicoextensive.ii -the particular construction illustrated, and the secondary emissive laminafiiz being; carried. by theelectrically conductivexdamina. 6.0.. The electrically conductive. lamina, 50. comprises a. very fine mesh metallic: creen; preferably a; woven wire screen: havi oi thenrder :of magnitude. of one;.iiundred;thou.-- sand. spaces: per square inch: Preferably... the;- screen. iszmade of wireiortheaorder of ma ni ude ei-fromrilioocl'to. :001 inch in diameter- ,TI'.he secondary emissive; l.amina. 61' is preferably formediby filling. the intersticesor spaces;ofi the; creen. with the .des red; secondary; emissive ma;- terial to form, in efiect;v verythindiaphragms, located in thespaces: of. the. screen andv Sup-1' pprted;by--.the wiresforming such spaces. Alternatively; ofqcourse. the; secondary emissive: lamina. maybe-somewhat thicker so. that the. metalliescreen. is, in effect, embedded therein.

The amplifier cell 50 maybe substitutedfor. the. amplifier cell. ll and. will operate in substan-. tially-ythe same manner so that a further de-- scription. thereof hereinis; thought; tobe un-.

necessary.

. Fig.1 6..=of .thedrawing illustrates. an application. otthepresent invention to a. television projec,-. tion system. which includes atypical projection tube forprojecting: an image; onto 1 a viewing;

screen-J1, which .may be either; the: indirect or the direct type. interposed between the; pro--- jection. 1111381110. and ;.the, viewingscreenr. 1 I: is a light. amplifier? which may incorporate either. the; amplifier-cell H or. the amplifier cell 5.0,. the light amplifier '12 being so. positioned that-its; photoemissive: lamina faces; the: projection tube 10. and. so. that. itsgcathodoluminescent lamina; faces the viewing screen 1 I. Interposed between the projection tube 10 and the light amplifier 1.2 is. a, lens'systema 13 for. projecting an image; from the. tube. onto the light amplifier. Similarly; interposedibetween the light. amplifier 12, and the-viewingscreenJ Us a lens system forprojectingthe amplified, image produced. by the light amplifier 12"onto1 the viewing screen i l 'Sinc.8,.- as.hereinbeforei discussed,. the light' amplifiery-lzamplifiesfi the. intensity of; the imagev many timesover, itwill. be apparent that: the areaofLthe vi win sc en Til maybe many times that of the tube. 10;.thereby Droviding; a large,

image readily viewable from. a distance. without any diminution of brightness. By way of .an example, if. we employthe amplification: factor ofrronephundred twentyfive hereinbefore. em.- ployed for illustrative purposes; it willbe ap parent: that; the area. of. theviewingscreen. H maywepproach one. hundred. twenty-five times: that of the tube 10 without'sacrificing anyof the. brightness. of the image projected by: thev tube.

In. addition to. permitting: the use of. a. large viewing-screen in a television projection system: without any; loss of? brightness, another. advantage of the present. invention is 1 that it permits;

the useof a relativelysmall cathode ray'tube 'lfl jection tube 80 which projects an image onto a; lem-xsystemt I; the lenssystemxlnzturn, project ingrthe image onto the -photoemi ssive lamina of. anenlarged .lightamplifier 82 of the invention.

In; thisprojection system, the cathodoluminesr cent lamina of thelight amplifier 82.serves as a.

viewing screen, The projection system illustrated in Fig. 7 has advantages similar to those. dis.- cussed in theprojection system of Fig. 6...

It will be understood that while we. have .dis-

without any reduction inthe brightness of the. image projected ontothe. screen. Reducing. the. sizesof projection lamps. in such systems has.

obvious advantages.

Also,. the lightamplii'ier of the present invention has possible application in fluoroscopy. For

fluoroscopic purposes, an amplifier cell similar'to.

the cell ll might be employed, except that it would be. necessary to add to the cell l.l a-.radioluminescent. lamina, not shown, between the photoemissive. lamina l 3 and the X-ray image.

Although wev have disclosed various exemplary embodiments of our inventionand have discussed a number of possible applications thereof for.

purposesgof. illustration, it will be understood that other embodimentsand applications of the invention. are possible without departing from the spirit thereof.

We claim :as. our invention:

1. In a radiationamplifier system, the com.-. bination of; a laminated cell of contacting lame inae including a. lamina of radiation-sensitive and electron-emissive material,. alamina of electron-sensitive and electron-emissive material. having the characteristic of emitting more than. one electron for each electron impinging thereon,

and a lamina of electron-sensitive and radiationemissive material, said laminae being arranged in the order specified; and means connected to said lamina of. electron-sensitive and electron-emissive material forisubjecting it to a positive potential.

2. A radiation amplifier system as. defined in.

claim lwherein said means includes: a lamina of electrically conductive material. carryingsaid lamina of electron-sensitive. and electron-emissive material; and a source of positive potential connected. to saidlamina of. electrically conductive material.

3. In aradiation amplifier-system thecombi nation of: a laminated cell of contactinglaminae:

including av lamina. of radiation-sensitive and electron-emissive material, alamina of electron sensitive and electron emissive' material having. the characteristic of. emitting more than one elec-= tron for each electron impingingthereon, and a lamina. of electron-sensitive and radiation-emissive material, said laminae-beingarranged in the order specified; meansconnectedto said lamina of electron-sensitive andv electron-.emissive material forapplying tell; a positive potential; and means encircling. said'cell forproducing a, mag--- netic field having lines of force; generally normal.

to said laminae.

4. In a radiation-amplifier, ala-minated cell of contacting laminaeincluding-a lamina, of radiation-sensitive. and electron-emissive material, a

plurality of laminae of electron-sensitive: and electron-emissivematerial-each having the char. acteristicof emittingmore than one electronfor each electron impinging thereon, and a lamina of electron-sensitive and radiation-emissive material, said laminae being arranged in the order specifi d.

5. A radiation amplifier system including a radiation amplifier according to claim 4 and including means connected to said laminae of electron-sensitive and electron-emissive material for applying progressively higher positive potentials to said laminae of electron-sensitive and electronemissive material in a direction irom said laminae of radiation-sensitive and electron-emissive material to said lamina of electron-sensitive and radiation-emissive material.

6. In a light amplifier system, the combination of: a laminated cell of contacting. laminae including a lamina of photoemissive material, a lamina of secondary emissive material, and a lamina of cathodoluminescent' material, said laminae being arranged in the order specified; and means connected to said lamina of secondary emissive material for subjecting it to a positive potential.

7. In a light amplifier system, the combination of: a laminated cell of contacting laminae including a lamina of photoemissivematerial, at least two laminae of secondary emissive material, and a lamina of cathodoluminescent material, arranged in the order specified; and means connected to said laminae of secondary emissive material for applying progressivelyjhigher positive potentials to said laminae of secondary emissive material in a direction from said lamina of photoemissive material to said lamina of cathodoluminescent material.

8. In a light amplifier system, the combination of: a laminated cell including a lamina of photoemissive material, at least two laminated elements each of which includes a lamina of electrically conductive material carrying a "lamina of secondary emissive material, and a lamina of cathodoluminescent material, arranged in contact in the order specified; and sources of progressively higher positive potentials connected to said 1aminae of electrically conductive material for applying progressively higher positive potentials to said laminae of electrically conductive material in a direction from said lamina of photoemissive material to said lamina of cathodoluminescent material.

9. A light amplifier system according to claim 8 wherein said lamina of electrically conductive material of each of said laminated elements comprises a sheet of metallic foil.

10. A light amplifier system according to claim 8 wherein said lamina of electrically conductive material of each of said laminated elements comprises a metallic screen, and wherein said secondary emissive material comprising the other lamina of said laminated element is disposed in the interstices of said metallic screen, whereby said laminae of said laminated element are substantially coextensive.

11. In a light amplifier system, the combination or: a laminated cell including a lamina of transparent material, a lamina of photoemissive material, at least two laminated elements each of which includes a lamina of electrically conductive material carrying a lamina of secondary emissive material, a lamina of cathodoluminescent material, and a lamina of transparent material, arranged in contact in the order specified; and positive potential sources connected to said laminae of electrically conductive material for applying progressively higher positive potentials to said laminae of electrically conductivematerial in a direction from said lamina of photoemissive material to said lamina of cathodoluminescent material.

12. In a light amplifier system, the combination of; a laminated cell of contacting laminae including a lamina of photoemissive material, at le'ast'two laminated elements each of which includes a lamina of electrically conductive material carrying a lamina of secondary emissive material, and a lamina of cathodoluminescent material, arranged in the order specified; sources of progressively higher positive potentials connected to said laminae of electrically conductive material in adirection from said lamina of photoemissive material to said lamina of cathodoluminescentmaterial; and means for limiting electron movement through said laminated cell to paths generally normal to said laminae.

13. In a light amplifier system, the combination of a laminated cell including a, lamina of photoemissive material, at least two laminated elements each of which includes a lamina of electrically conductive material carrying a lamina of secondary emissive material, and a lamina o! cathodoluminescent material, arranged in contact in the order specified;sources of progressively higher positive potentials connected to said laminae of electrically conductive material in a direction from said lamina of photoemissive material to said lamina of cathodoluminescent material; and electromagnetic means, including a helical coil encircling said laminated cell and a source of electric current connected to said coil. for producing an electromagneticfield having lines-of force generally normal to said laminae.

14. In a light amplifier system, the combination of: a-- laminated cell of contacting laminae including 'a lamina of photoemissive material, at least two laminae of secondary emissive material, and a lamina of cathodoluminescent material, arranged in the order specified; sources of progressively higher positive potentials connected to said laminae of secondary emissive material in a direction from said lamina of photoemissive material to said lamina of cathodoluminescent material; and electro-magnetic means for limiting electron movement through said laminated cell to paths generally normal to said laminae.

15. A viewing screen comprising the light amplifier system defined in claim 11.

LEE ms FOREST. WILLIAM A. RHODES.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

